The present invention relates generally to device processing and, in particular, to the planarization of non-planar surfaces in the fabrication of semiconductor or other devices.
The fabrication of electronic devices, such as semiconductor devices, typically produces surfaces that are non-planar. In some fabrication techniques, for example, multiple layers are formed sequentially on a silicon or other substrate. The layers can be processed into a desired pattern by the selective deposition of one or more materials or by removing selected regions of one or more layers. As more layers are formed, the topography or irregularity of the surface tends to increase.
An irregular surface can present various difficulties during device fabrication. For example, lithographic processes are often used to provide the patterning for the layers. During a typical lithographic process, a photosensitive material is deposited on a layer, exposed to radiation in a desired pattern, and developed to reveal the exposed pattern. To produce a photolithographic pattern with high resolution, the exposure light is focused at a specific depth, and the focus is maintained over a range of depths that is at least about two times the width of the pattern features. However, if the deposited resist material is not planar, the exposed image will not be in focus across the entire semiconductor wafer and throughout the film thickness. The likelihood of degraded lithographic results increases. Moreover, surface irregularities adversely affect metallization and other interconnections because the metal deposited over the surface has bends and turns which conform to the surface irregularities. Such bends and turns can cause undesirable current crowding.
To reduce the effects of surface irregularities, techniques are used to planarize the surface of the device on which the photosensitive material is deposited. Such techniques include, for example, etch-back techniques as well as chemical-mechanical polishing (CMP) techniques.
Recently, a technique has been proposed in which an object with a flat surface is pressed into contact with a planarization material, such as a photoresist or resin, and deposited on the wafer. The planarization material is cured while in contact with the flat surface, and the flat surface is separated from the planarization material. The planar surface then can be transferred from the planarization material to the underlying layers.
One difficulty encountered with the foregoing technique is that, when the flat surface is separated from the planarization material, the planarization material can adhere to the flat surface, resulting in a surface that is less planar than desired. Accordingly, it would be advantageous to have a technique for planarizing a substrate in a manner that reduces or prevents adhesion of the planarization material to the flat surface.
In general, according to one aspect, a method of planarizing a surface of a wafer includes providing a planarization material on the wafer surface and bringing a substantially flat surface into contact with the planarization material on the wafer. The planarization material is exposed to radiation at a first wavelength to cure the planarization material and is exposed to radiation at a second wavelength to cause changes to the planarization material that facilitate separation of the flat surface from the planarization material.
The flat surface can be provided, for example, by using an optical flat that is transparent to the radiation at the first and second wavelengths. The radiation at the first and second wavelengths can be transmitted through the optical flat and subsequently absorbed by the planarization material. The planarization material can include, for example, a resist or resin.
In various implementations, one or more of the following features are present. In one embodiment, curing of the planarization material is inhibited at or near the interface with the flat surface to facilitate separation of the planarization material and the flat surface. The planarization material can include an agent which is sensitive to the radiation at the second wavelength and which inhibits curing of the planarization material when exposed to the radiation at the second wavelength. Preferably, the radiation at the second wavelength is absorbed substantially by the planarization material at or near the interface between the planarization material and the flat surface. In one particular implementation, the planarization material includes an acid-generating agent which is sensitive to radiation at the first wavelength and a base-generating agent which is sensitive to radiation at the second wavelength. The base generated at or near the interface when the planarization material is exposed to radiation at the second wavelength can be used to neutralize acid produced when the planarization material is exposed to radiation at the first wavelength.
In a second embodiment, the planarization material can produce a gaseous by-product when exposed to radiation at the second wavelength. The gaseous by-product can produce localized pressure at the interface between the planarization material and the flat surface. The localized pressure can facilitate separation of the planarization material from the flat surface. In one particular implementation, the planarization material includes a carbonate which generates a gas such as carbon dioxide when exposed to deep ultraviolet radiation.
In a third embodiment, the planarization material can include an agent which is sensitive to the radiation at the second wavelength and which causes the planarization material to decompose when exposed to the radiation at the second wavelength. Preferably, the radiation at the second wave-length is absorbed substantially by the planarization material at or near the interface between the planarization material and the flat surface. For example, in one particular implementation, the planarization material can include a polymer with a doubly substituted carbon backbone and the second wavelength can be in the deep UV range.
The foregoing techniques can be combined to facilitate further separation of the planarization material from the optical flat.
In general, improved planarization is possible by using the techniques described herein. Higher degrees of planarization are made possible even for devices with severe topography.
Other features and advantages will be readily apparent from the following detailed description, the accompanying drawings and the claims.